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Choose the Spring integration that fits your application
Google Cloud documents three Spring approaches for messaging with Pub/Sub. They differ mainly in abstraction level and how naturally they fit an existing application architecture.
| Option | Best fit | Control and trade-off |
|---|---|---|
| Spring Cloud GCP Pub/Sub Starter | A conventional Spring Boot service that needs to publish and consume Pub/Sub messages. | The most direct Spring Boot path: it auto-configures Pub/Sub components and still allows use of the Google Cloud Java client for advanced scenarios. The starter abstraction does not expose the acknowledgment response interface required for the Java client’s exactly-once acknowledgment feature. |
| Spring Integration channel adapters | An application whose messaging is already organized as Spring Integration channels and flows. | Fits a channel-based topology. Choose it when that integration model is useful to the application rather than adding it solely to send a message. |
| Spring Cloud Stream Binder | An application already structured around Spring Cloud Stream bindings. | Fits the Spring Cloud Stream model. Select it when bindings are the natural interface for the service, rather than introducing the model for a single Pub/Sub connection. |
For a new or straightforward Spring Boot integration, start with the starter. You can use the Java client alongside the Spring components when you need lower-level behavior that the starter abstraction does not expose.
Add the Pub/Sub starter and configure the environment
Add the dependency
Use the official starter coordinate com.google.cloud:spring-cloud-gcp-starter-pubsub with the Spring Cloud GCP BOM. You can add it through your Maven or Gradle build, or select GCP Messaging in Spring Initializr. Use the BOM to keep Spring Cloud GCP dependency versions aligned; the documentation referenced here does not establish a particular version number.
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Set project and credential configuration
Configure the Google Cloud project and authentication through Spring Cloud GCP properties. The settings to account for include project ID, credential source (a credentials location or encoded key), OAuth scope, whether the integration is enabled, and an emulator host when developing locally. Keep these values environment-specific: local emulator settings and production project credentials should not be accidentally interchanged.
Provide the Pub/Sub resources
The service needs a topic to publish to and a subscription to consume from. Create or select those resources in the intended project, then publish and receive using the starter abstractions or use the Java client where finer control is required. A topic is the publishing destination; subscribers receive messages through subscriptions, so verify that the subscription is attached to the topic your publisher uses.
Test locally with the Pub/Sub emulator
The Pub/Sub emulator lets you develop and exercise messaging flows without sending messages to a live Google Cloud Pub/Sub service. Start it with the Google Cloud CLI, then configure the Spring integration’s emulator-host setting to reach it. The emulator commonly listens on port 8085; confirm the host and port used by your local setup rather than assuming that value applies in every environment.
Resources created in the emulator exist only for that emulator session. Do not treat emulator topics, subscriptions, or messages as durable test fixtures across restarts.
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The emulator supports publishing, pull and push delivery, ordering, replay, dead-letter forwarding, retry policies, Avro schemas, and filtering. It does not support IAM operations and has incomplete retention and expiration behavior. That makes it useful for local integration work, but not a substitute for validating production permissions or every operational behavior against the live service.
Understand acknowledgments, redelivery, and exactly-once delivery
Pub/Sub provides at-least-once delivery by default. A message may be delivered again when processing or acknowledgment does not complete as expected, so consumers should be safe to retry. Make message handling idempotent, and acknowledge only after the work and its important side effects have been durably completed.
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Exactly-once delivery is available only for pull subscriptions, including subscribers using StreamingPull. Push and export subscriptions do not support it. Exactly-once is regional and can increase publish-to-subscribe latency, so the delivery mode is a choice about correctness requirements, topology, and responsiveness—not a free switch for every subscription.
There is an additional Spring-specific constraint: the Spring Cloud GCP abstraction does not expose AckReplyConsumerWithResponse, which the Java client uses for the exactly-once acknowledgment feature. If your application must use acknowledgment responses, use the underlying Java client path and verify that the library version you deploy supports the required behavior.
Use ordering keys only where sequence matters
Pub/Sub ordering is per ordering key, not global. Give messages that must remain in sequence the same key, publish messages for that key in one region, and enable message ordering on the subscription. Messages carrying different keys have no ordering relationship with one another.
Ordering trades throughput and latency for sequencing. Under Google’s documented ordering model, a key can be up to 1 KB and publishing throughput for one key is limited to 1 MBps. A key that attracts more work than subscribers can process can become a hot key and build backlog; monitor per-key workload rather than assuming that adding unrelated keys changes the order of messages within a busy key.
Choose a delivery mode and prepare for production
Pull, StreamingPull, and push differ in how the subscriber receives work and manages its processing loop. The decisive constraint is that exactly-once is supported only by pull and StreamingPull. Evaluate the remaining choice against your acknowledgment needs, latency targets, ordering requirements, and regional design.
Quick Recap
- Make processing idempotent. A retry or redelivery should not cause an irreversible duplicate side effect.
- Acknowledge after durable work. Do not acknowledge a message before the processing result that matters to your application is safely recorded.
- Keep each ordered key in one region. Track backlog for keys whose traffic could outpace subscriber processing.
- Plan retries and dead-letter handling deliberately. Exercise those behaviors in the emulator, then validate production behavior against Google Cloud because emulator support is incomplete.
- Separate environment configuration. Set project ID, credential source, and emulator host for the environment in which the service runs.
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